The effect of parasitic invasion by the Rhizocephala Sacculina ignorata on brachyuran crabs Actaea hirsutissima was studied. The organic constituents (glucose, total proteins, albumin, triglycerides and cholesterol) of the haemolymph of the infected and normal crab Actaea hirsutissima were estimated. A significant reduction in serum protein and albumin was observed in parasitized individuals; while serum glucose, cholesterol and triglycerides concentrations were significantly increased. Parasitism by S. ignorata dramatically affects the serum chemistry of infected crabs. T-test values were statistically highly significant between healthy and infected specimens.